1. Field of the Invention
The invention relates to a method of producing epoxides [oxiranes; literally "olefin oxides"] with 2 to 6 C atoms, especially propene oxide, by gas-phase epoxidation of the corresponding olefin with 2 to 6 C atoms with hydrogen peroxide in the presence of a solid catalyst.
2. Background Information
The epoxidation of olefins such as propene using hydrogen peroxide is successful in the liquid phase using a titanium silicalite catalyst--see U.S. Pat. No. 5,874,596 and DE 197 31 627. A disadvantage of this method is the rapid deactivation of the catalyst by high-boiling byproducts.
The liquid-phase epoxidation of olefins with hydrogen peroxide in the presence of a catalyst containing molybdenum or tungsten is also known--see Weigert et al., Chem.-Ztg. [German=Chemische Zeitung] 99, 19 (1975). The workup of the reaction mixture and the recovery of the catalyst are quite expensive.
The carrying out of the epoxidation in a membrane reactor is also known from CA 2,206,626 A1, in which catalytically active particles are intercalated [inserted] in the composite membrane and a gas phase with the olefin such as propene to be epoxidized is located on the one side of the membrane and on the other side a liquid phase with hydrogen peroxide is located. The catalytically active particles preferably consist of titanium silicalite.
Instead of in the liquid phase, ethylene can also be epoxidized in the gas phase in the presence of a silver-containing catalyst at 200 to 300.degree. C. In this instance, air or molecular oxygen is used as epoxidizing agent instead of hydrogen peroxide (see, e.g., U.S. Pat. No. 4,374,260).
The attempt has also been made to epoxidize lower olefins with hydrogen peroxide in the gas phase, during which hydrogen peroxide is thermally or catalytically activated: Thus, according to G. M. Mamedjarov and T. M. Nagiev (Azerb. Khim. Zh. [Russian=Azerbaizhanskii Khimicheski Zhurnal] ethene and propene can be epoxidized at 500 to 600.degree. C. in a gas phase in the absence of a catalyst. Epoxide yields, that were initially low, were able to be increased by T. M. Nagiev et al. (Neftekhimiya [Russian] 31 (1991), 670-675) by optimization to approximately 50 to 55%. The high reaction temperatures, that oppose an economic process, are disadvantageous.
H. M. Gusenov et al. (Azerb. Khim. Zh. (1984), 47-51) investigated the mechanism of a similar method; however, the gas-phase epoxidation takes place here in the presence of a Si-containing catalyst at 425 to 500.degree. C. Propene and vaporous hydrogen peroxide are supplied to a tubular reactor; the conversion of propene is in a range of 15 to 65%.
T. M. Nagiev et al. (Neftekhimiya 31, (1991), 670-675) teach an improved gas-phase epoxidation in the presence of an Fe-containing catalyst: Propene is epoxidized with hydrogen peroxide to propene oxide using magnetite as catalyst at approximately 250.degree. C. with a yield of about 30%. However, the catalytic service life of 25 h is very low. A longer service life and a further lowering of the reaction temperatures can be obtained with an Fe.sup.III OH-protoporphyrin catalyst bound to aluminum oxide as carrier. A propene oxide yield of approximately 50% is obtained with this catalyst at a temperature of about 160.degree. C. and a molar dosing ratio of C.sub.3 H.sub.6 :H.sub.2 O.sub.2 :H.sub.2 O=1:0, 2:0,0.8.